In general, a large amount of iron is contained in a waste non-ferrous slag discharged from both a pyrometallurgical process for smelting or refining concentrated copper and concentrated lead and a pyrometallurgical process for re-treating a residue discharged from a hydrometallurgical process of zinc. It is known that the content of iron to be recovered varies depending on the pyrometallurgical process for smelting the non-ferrous metal, but about 35-45 wt % of iron is contained in the waste non-ferrous slag.
Nevertheless, since the total content of non-ferrous metals including copper, zinc and lead leading to a deterioration of hot brittleness of iron or steel, which are contained in the waste non-ferrous slag, is more than 4 wt % to date, the waste non-ferrous slag is not used as an iron or steel raw material.
Thus, as conventional arts, although technologies for recovering iron from an iron or steel slag have been developed variously, a technique for recovering iron from a waste non-ferrous slag is not carried out.
Korean Patent Laid-Open Publication No. 2005-76556 discloses a method for recovering iron and manufacturing iron powder from a granulated water-quenched blast furnace slag. This invention is directed to a method that removes iron contained in a granulated water-quenched blast furnace slag, functioning as a factor influencing a deterioration in quality of the slag due to incomplete removal of iron from the slag generated during a pig iron manufacture in the iron manufacture process, and uses the same. In particular, the above Korean Patent invention is directed to method in which iron particles dispersed in the water-quenched blast furnace slag are subjected to magnetic separation to directly use the resulting magnetic iron concentrates as high-priced iron powder raw materials, or the iron particles are crushed and reduced to manufacture a high-value sponge-type iron powder. However, this method does not teach a solution to a problem of non-ferrous metals leading to a deterioration of hot brittleness of iron or steel as the characteristics of the waste non-ferrous slag, and cannot solve a problem in that iron components contained in the waste non-ferrous slag has a very low reduction potential as being bound to the non-ferrous metals in an amorphous state.
In addition, Korean Patent Laid-Open Publication No. 2002-36075 discloses a technology that extracts valuable metals from a non-ferrous metal sludge. This patent invention describes a technique for extracting a small amount of valuable metals contained in a non-ferrous metal sludge disposed of at industrial fields, but does not teach any treatment of large amounts of iron components contained in an amorphous state in a waste non-ferrous slag.
Moreover, Korean Patent Laid-Open Publication No. 2002-36075 discloses a method for separating iron from a slag generated as a by-product in a steel making process. The above patent describes a method that subjects a powered slag is subjected to magnetic separation according particle size to recover iron from the slag, thereby increasing the iron recovery rate, but does not teach any treatment large amounts of iron components contained in an amorphous state in the waste non-ferrous slag.
In the meantime, 2 tons of a waste non-ferrous slag per ton of produced copper is generated in a pyrometallurgical process for copper concentrate, 0.45 ton of a waste non-ferrous slag per ton of produced lead is generated in a pyrometallurgical process for lead concentrate, and 0.2 ton of waste non-ferrous slag per ton of produced zinc is generated in a pyrometallurgical process for re-treatment of a residue discharged in a hydrometallurgical process for zinc. This results in an environmental pollution, which causes a social problem.
However, a large amount of iron contained in the waste non-ferrous slag as an industrial waste is a resource that is too precious to be thrown away as a waste. Thus, it is required that iron should be recovered in terms of resource recycling in the state economy. Recovery and recycling of iron will be very useful in terms of effective utilization of resources.
A method for recovering iron from the waste non-ferrous slag as the industrial waste is roughly classified into a physical separation process and a pyrometallurgical process.
The physical separation process is a method that crushes a waste non-ferrous slag and subjects the crushed material to an oxidation reaction, followed by magnetic separation, thereby separating and recovering iron from the waste non-ferrous slag. However, such a physical separation process entails a shortcoming in that since iron oxides contained in the waste non-ferrous slag is very low in reactivity, it is very difficult for the crystal structure of the waste non-ferrous slag to be changed by the oxidation, so that the separation and recovery rate of iron is very low, which is less than 50%, and the recovered iron concentrate contain large amounts of non-ferrous metals including copper, zinc and lead contributing to a deterioration of hot brittleness of iron or steel as follows: copper is 1%, zinc is 2%, and lead is more than 1%. For this reason, in order to use the iron concentrate recovered by the physical separation process as an iron or steel raw material, the iron concentrate needs to be re-treated. Thus, the physical separation process of crushing a waste non-ferrous slag and subjecting the crushed material to an oxidation reaction, followed by magnetic separation has not been commercialized yet.
On the other hand, the pyrometallurgical process is a method that mixes a waste non-ferrous slag with fluxes as various slag composition regulators such as alumina (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), and silica (SiO2), and carbon as a reducing agent using a melting furnace, and then melts the mixture at a high temperature of about 1550˜1600° C., which is more than a melting temperature of iron, thereby recovering iron from the waste non-ferrous slag. However, such a pyrometallurgical process has a shortcoming in that it is disadvantage in terms of cost effectiveness due to the use of fluxes as various slag composition regulators, the control of the process is difficult, and the process temperature is high. In addition, this pyrometallurgical process entails a disadvantage in that since non-ferrous metals including copper, zinc and lead leading to a deterioration of hot brittleness of iron or steel, which are contained in the waste non-ferrous slag, are recovered together with iron in a larger amount than in the above-mentioned physical separation process, a complex re-treatment is required for the waste non-ferrous slag to be used as an iron or steel raw material. Thus, any pyrometallurgical process of recovering iron from the waste non-ferrous slag as an industrial waste has not been commercialized yet.
Meanwhile, a method for recovering iron from an iron or steel slag containing 10-25 wt % of iron but not containing non-ferrous metals including copper, zinc and lead is largely classified into a physical separation process and a pyrometallurgical process. Such a method is relatively very simple technically as compared to the method for recovering iron from the waste non-ferrous slag as an industrial waste because it does not require a step of removing non-ferrous metals including copper, zinc and lead.
Accordingly, the present inventors have made extensive efforts to solve the above-described problems occurring in the prior art and, as a result, have found that a waste non-ferrous slag discharged from a process for smelting of non-ferrous metals including copper, zinc and lead is crushed, the crushed waste non-ferrous slag is mixed with carbon as a reducing agent and calcium carbonate (CaCO3) as a reaction catalyst, the mixture is subjected to a reduction reaction at a temperature below a melting temperature of iron, thereby converting amorphous iron oxides, bound to alumina (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), silica (SiO2), zinc oxide (ZnO), copper oxide (CuO) and lead oxide (PbO) in the waste non-ferrous slag, to crystalline iron (Fe) and iron carbide (Fe2C); the resulting material is crushed to separate iron and iron carbide from components such as alumina (Al2O3), calcium oxide (CaO), magnesium oxide (MgO), silica (SiO2), zinc oxide (ZnO), copper oxide (CuO) and lead oxide (PbO); the crushed material is separated into fractions by particle size; and the fractions are subjected to wet magnetic separation and dry magnetic separation to separate and recover magnetic iron concentrates in which the total content of non-ferrous metals including copper, zinc and lead is less than 1% from the fractions, so that the recovered iron concentrates can be used as iron or steel raw materials without being re-treated. Based on this finding, the present invention has been completed.